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How does ejaculation work?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

January 6, 2009

Dear Straight Dope:

What exactly causes ejaculation? Is the brain sent some sort of pleasure signal that it relays back to the muscles in the region? Is there a threshold after which ejaculation occurs in every male? Or does continuous rubbing cause a pressure buildup in some way?

Peter, Delaware

Gfactor replies:

A reasonable set of questions, Peter. We’ll take them in order.

What exactly causes ejaculation?

Perhaps surprisingly, that’s a tough one – the “exactly” part being the major sticking point. Scientists are pretty sure it has something to do with the spine and some muscles, but they’re not sure how it all fits together. Part of the problem is a lack of careful research. Physiologist Roy Levin reviewed the literature in 2005 and discovered that, “of the four “e’s” of male sexual function, excitation, erection, emission, and ejaculation, the mechanism that has been studied the least is the last.” Standard Practice in Sexual Medicine (2006) says, “A detailed, non-disputed physio-anatomic description of the mechanism of human ejaculation has still to be produced.” In other words, we don’t know everything just yet – we’re still scratching our heads a bit. The fact that we don’t know everything, of course, doesn’t mean we know nothing.

Another part of the problem is one of definition. Scientists can’t even agree how to define orgasm, let alone appreciate all of its nuances.

Is the brain sent some sort of pleasure signal that it relays back to the muscles in the region?

This is a good start. We actually know a lot about this. The answer is no – sort of. Ejaculation is a spinal reflex; it can be completed without the brain being involved. Men with spinal cord injuries can still ejaculate, even though their brains receive no “pleasure signals.” And recent research has isolated an area of the spinal cord containing what are known as the lumbar spinothalamic neurons, electrical stimulation of which causes ejaculation, or at least it does for the rats they’ve tried it on.

The tricky part is that there’s also a cerebral component to male orgasm – men can, to some extent, voluntarily resist ejaculation. This aspect of the process is not as well understood. Brain imaging studies haven’t turned up a single area of the brain that seems to be at work during ejaculation – or, more to the point, right before ejaculation. And even if we could locate an orgasm center in the brain, we’d still need to determine the relationship between this area and what otherwise seems to be a reflex.

Is there a threshold after which ejaculation occurs in every male?

Yes, but again it’s difficult to identify what the trigger is. It’s important to understand that ejaculation has two separate phases: emission, in which semen is deposited in the urethra, and propulsatile ejaculation, or ejaculation proper, in which the semen leaves the urethra in a hurry. What we’re concerned with here is what makes propulsatile ejaculation bound to happen. The working hypothesis, called the “pressure chamber” theory, is outlined in The Science of Orgasm (2005) by Barry Komisaruk and colleagues.

But little is known about how this all actually works. For a while, the theory was that distension of the prostatic urethra triggered the feeling of inevitability. Levin, however, presents four pieces of evidence arguing against this notion. First, subjects can be induced to have dry ejaculations, in which their bodies go through the motions minus any fluid buildup. Second, ultrasound studies showed no “pressure chamber” in subjects who ejaculated. Third, a series of experiments with a balloon catheter suggest that the small volume of semen in the ordinary ejaculation would not be enough to trigger such a reflex. Fourth, scientists found that while rats whose urethras had been numbed with a lidocaine lavage did not ejaculate when the urethra was artificially stimulated, but they did during ordinary coitus – if your definition of ordinary coitus includes copulating while hooked up to a bunch of electrodes and surrounded by leering grad students. At any rate, the rats got off even though their urethras were insensate, meaning that in rats at least some other mechanism must be at work.

In fact, scientists aren’t even sure what role the urethra plays in the physical sensation of orgasm – the evidence is confusing. Levin reports at least one case of a man rating his dry orgasm as a 9 out of 10 in terms of pleasure, but notes that another study described dry orgasm as “a ghostly echo of a remembered orgasm.”

Or does continuous rubbing cause a pressure buildup in some way?

Probably not, as rubbing isn’t a prerequisite for male orgasm. Adolescent boys have nocturnal emissions, after all, and men without sensation in their genitals can still have orgasms. Continuous rubbing does seem to stimulate mechanoreceptors in the glans penis, which triggers a reflex called the glans-cavernosovesicular reflex. But this can’t be the only factor in play. In addition to cases like those mentioned above, there is electroejaculation, which is accomplished via anal stimulation with an electric probe. However, a recent study notes that this sort of ejaculation is less dramatic than orgasms achieved the usual way: “Anterograde ejaculate is not produced in a projectile fashion, but rather as an intermittent release of semen during the course of the procedure. Between 15 and 35 stimulations are usually needed to ensure emptying of the semen.”

While research to date on ejaculation is inconclusive, we’re finding out more all the time. For example, a recent letter to the journal Medical Hypotheses suggests that ejaculation might relieve nasal congestion. The people at Kleenex must be thinking: one way or another, they’ll still need us.

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STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL'S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU'D BETTER KEEP YOUR FINGERS CROSSED.